Aims: The cytochrome P450 enzyme CYP2C9 catalyses the 4'-hydroxylation of the nonsteroidal analgesic drug diclofenac in humans. We studied the influences of the known amino acid variants, CYP2C9*2 (Arg144Cys) and CYP2C9*3 (Ile359Leu), on diclofenac pharmacokinetics after a 50-mg oral dose of diclofenac in healthy volunteers. As a surrogate marker of diclofenac activity, the ex vivo formation of prostaglandin E2 and thromboxane B2, which reflects COX-2 and COX-1 activity, was measured.

Methods: Genotyping was performed in 516 healthy volunteers to obtain 20 participants with all allelic combinations of the two CYP2C9 variants Arg144Cys (*2) and Ile359Leu (*3). Diclofenac and 4'-hydroxydiclofenac were quantified in plasma by reversed phase h.p.l.c. after oral intake of 50 mg diclofenac. Concentrations of thromboxane B2 (TxB2) and prostaglandin E2 (PGE2) were measured by immunoassays.

Results: There was no evidence of impaired metabolism of oral diclofenac in heterozygous and homozygous carriers of the CYP2C9 alleles *2 and *3 compared with the wild type (mean CL/F (95% CI) 20.5 (11, 30) l h-1 for *1/*1, 29.9 (19, 40) l h-1 for *1/*2, 30.0 (4, 56) l h-1 for *2/*2, 22.6 (12, 33) l h-1 for *1/*3, 23.5 (11, 37) l h-1 for *3/*3 and 37.3 (-15, 89) l h-1 in *2/*3). Furthermore, plasma concentrations of the metabolite 4'-hydroxydiclofenac were not lower in carriers of the CYP2C9 low-activity alleles *2 and *3 compared with carriers of the CYP2C9*1/*1 genotype. Marked diclofenac mediated inhibition of COX-1- and COX-2 activity was detected in all individuals independent of CYP2C9 genotype.

Conclusions: Polymorphisms of the CYP2C9 gene had no discernible effect on the pharmacokinetics and pharmacodynamics of diclofenac. The question of whether enzymes other than CYP2C9 play a major role in diclofenac 4'-hydroxylation in vivo or whether 4'-hydroxylation is not a rate-limiting step in diclofenac elimination in vivo, or whether the effect of the CYP2C9 polymorphisms is substrate-dependent, needs further investigation.